378 PAST succession: the ceneosere. 



be seen in the characteristic hydroseres and xeroseres respectively of these two 

 regions. Climatically, postglacial time appears to have been far from imiform, 

 as Huntington seems to have convincingly shown. While it is probably true 

 that the present is too remote from the Pho-Pleistocene deformation to be 

 further subject to it, it is clear that it will continue to show climatic and 

 topographic changes of the same kinds but of less intensities, as well as to 

 produce a myriad of the biotic changes typical of a human period. Thus, the 

 exact student of present-day succession is necessarily, even though uncon- 

 sciously, a student of past succession as well. 



THE PEAT CLISTASE. 



No other phase of succession has received so much attention as that which 

 has to do with the clisere of glacial and postglacial times. This has become 

 a field by itself, in which the methods of ecology and geology have been 

 combined to form the foundation of paleo-ecology. The investigations for 

 the most part have been primarily geological and historical, and the basic 

 importance of sucoessional methods and principles has not been generally 

 recognized. The number of articles in this field has been very large, and many 

 of them are chiefly theoretical or controversial in character. As a consequence 

 the following summary is not intended to be exhaustive. Papers which deal 

 with the floristics of different horizons, or with the floristic aspects of glacial 

 and postglacial invasions, have been ignored for the most part, though a 

 number of them will be found in the bibhography. Those which follow have 

 been selected primarily because of their indication of successional relations, 

 especially with respect to the peat costases and clistases, which have preserved 

 the record of the coseres and cUseres of the Pleistocene and Human periods. 

 Practically all of the articles considered are European, since American students 

 have barely touched this field as yet. A few papers on American stases are 

 appended. The classical investigations of Steenstrup, Vaupell, Nathorst, and 

 Blytt have already been touched upon in Chapter II. 



Blytt (1876) based his theory of the immigration of the Norwegian flora in 

 alternate wet and dry periods chiefly upon the following facts: 



(1) Forests formerly occurred much further north than at present; (2) the 

 species of the peat-beds vary with the elevation above the sea-level, but are 

 similar at the same altitude; (3) peat-beds of the lowest elevation contain 

 but a single layer, and the number of layers increases with the altitude. Nor- 

 way has been elevated about 200 m. since the glacial period. The glacial 

 Yoldia clay at 116 to 180 m. constitutes the bottom of the highest peat-moors, 

 which contain the remains of three different forests. The lowermost layer 

 contains Pimis silvestris and Betula, the next Quercus, Alnus, Corylus, and 

 Prunus, the uppermost Finns silvestris again. The moors from 50 to 60 m. 

 contain only the two upper layers, as the region was under water while the 

 lowest layer was being formed. Finally, the moors at 16 m. of elevation 

 contain only the upper layer of Firms silvestris. Blytt assumed the sequence 

 of events to have been as follows: An increase of rainfall caused the swamping 

 of forest areas and the death of the trees, with the result that their fallen 

 trunks became embedded in the peat. A second increase of water flooded 

 the moors and caused the appearance of water-plants. With the coming of 

 a dry period these disappeared and Sphagnum again developed and formed 

 hummocks, which became drier and permitted pines and birches to invade, 



